What does the compression setting do to a PNG?
No, the compression slider does not affect the image quality.
PNG uses a lossless compression algorithm called DEFLATE (among some other things), which is the same as is used to produce .zip files.
Long story short, the only difference between PNG files with 0% and 100% compression is the CPU time it takes to perform the compression (and of course the resulting file size).
Some tests:
Saving a 1920x1080 render of the default cube:
0% compression:
0.66 seconds, 8116 kilobytes (8 megabytes)
Zipping an uncompressed png produced a 48 kilobyte file.
100% compression:
0.86 seconds, 36 kilobytes
That's a compression ratio of 225 (!) and a time difference of .2 seconds.
Saving a pure high-frequency noise image (also 1920x1080):
0% compression:
.9 seconds, 8116 kilobytes (8 megabytes)
Zipping an uncompressed png produced a 5496 kilobyte file.
100% compression:
2.01 seconds, 5108 kilobytes (5 megabytes)
That's a compression ratio of 1.58 and a time difference of 1.11 seconds.
All tests were conducted with blender and compression times measured from the &quot;saving&quot; time printed to stdout and averaged over five samples.
As you can see, more uniform images (like the grey flatness of an empty blender scene) are much more compressible than highly variable images (such as pure RGB noise). The time it takes to perform the compression isn't very significant even in a worst-case scenario, however that 1 extra second could add up when rendering an animation.